In recent years, the 3GPP has conducted research on picocells with a view to distributing the load of macrocells. The heterogeneous network environment where macrocells coexist with picocells has attracted much attention because it can improve system performance even further compared with the existing macrocell environment.
To efficiently distribute the load of macrocells over picocells, the 3GPP has introduced Enhanced Inter-Cell Interference Coordination (eICIC).
In general, picocells are lower in transmit power and antenna height compared with macrocells. Hence, when the rule of association between base station (ENB) and user equipment (UE) (a UE is served by the ENB with the highest signal strength) is applied, the load of macrocells may be not sufficiently distributed over picocells.
In other words, when each UE selects a cell with the highest Reference Signal Received Power (RSRP) as the serving cell, some UEs may connect to the macrocell although a picocell is the best cell. Such UEs may cause severe interference to the picocell, degrading overall network performance. In addition, when the number of UEs connected to the picocell is much smaller than the number of UEs connected to the macrocell, the efficiency of resource utilization may become very poor.
To address the above problem, eICIC provides Cell Range Expansion (CRE) to set a criterion for handover between macrocell and picocell. CRE enables higher user offloading from the macrocell on to picocells by requiring a UE to preferentially select a picocell if the received signal strength from the picocell is less than that from the macrocell by a preset CRE bias (dB).
However, UEs in the CRE zone connecting to the picocell can suffer from severe interference from the macrocell since the RSRP of the macrocell is higher than that of the picocell for such UEs.
To guarantee signal quality for UEs that would not receive a service from the picocell if the CRE bias were 0 dB (referred to as a CRE UE), the macrocell may reduce interference to the picocell by not transmitting data at a specific subframe. Such a subframe at which the macrocell does not transmit data is referred to as an Almost Blank Subframe (ABS). To enhance performance of the overall network including macrocells and picocells, it is necessary to appropriately set the ratio of ABSs to all subframes (ABS ratio).
In the related art, the ABS ratio may be determined on the basis of the number of UEs connected to the macrocell and the number of UEs connected to the picocell. The ABS ratio determined in this way may be appropriate when individual UEs need the same amount of radio resources.
However, determining the ABS ratio based on the number of macrocell UEs and picocell UEs may be inappropriate when UEs generate different amounts of traffic or need different amounts of radio resources owing to different channel quality levels. For example, when the macrocell serves five UEs needing 50 resource blocks (RB) per subframe and the picocell serves five UEs needing one RB per subframe, if the ABS ratio is determined based on the number of UEs, the ABS ratio may be set to a value larger than necessary although the number of subframes needed by the macrocell for data transmission is larger than that needed by the picocell.
Accordingly, it is necessary to provide a scheme for controlling interference in a heterogeneous network system including macrocells and picocells by efficiently determining the ABS ratio in consideration of overall network performance.